The invention relates to improved tungsten-carbide dies made by metal injection molding (xe2x80x9cMIMxe2x80x9d).
Tungsten-carbide dies are currently made from cylindrical blanks produced by the press and sinter method known as Powder Metallurgy or xe2x80x9cPM.xe2x80x9d Cobalt, in various volume percentages, is blended with tungsten-carbide. A mixture of various powders are used in the process. Our process allows us to make our dies with lower percentages of cobalt (which is an advantage in itself because cobalt is expensive). This results in increased hardness and abrasion resistance when compared to dies with higher cobalt content. It is also possible to add other metals and alloys to our feedstock to give the resulting metal improved characteristics and performance.
Powder Metallurgy (xe2x80x9cPMxe2x80x9d) uses oblong or shard-shaped powders for various reasons. To begin with, they are typically less expensive than spherical powders. More importantly, spherical powders do not work well (if at all) in PM. When the tungsten-carbide and cobalt powders are pressed into the cylindrical die, they are compressed, which gives the part its stability during the sintering process. The shard particles of various sizes, xe2x80x9cinterlockxe2x80x9d to a certain extent. Pressing spherical powders in a PM process does not provide that interlocking.
Further, the use of spherical powders would substantially exacerbate the deformation that occurs during the sintering of PM parts. The deformation is caused primarily when the cobalt particles melt and fall through the spaces between the tungsten-carbide particles. Such deformation is already a significant problem in producing tungsten-carbide dies by PM.
In the PM process, a selected powder is pressed into a die or mold at high pressures. The pressed part is then sintered at high temperature to fuse the powders into xe2x80x9csolidxe2x80x9d metal. The part is not really solid, however. It has porosity, which is measured as its density (expressed as a percentage of the theoretical 100% density of wrought metal).
It is well known in the PM field that, in general, increasing the density of a sintered powdered metal item (i.e. reducing its porosity) will significantly improve its strength and durability. At high levels of porosity (i.e. low density), the metal is brittle and of low fatigue strength. Accordingly, considerable effort is expended (and significant cost incurred) in trying to increase the density of the PM blanks, which typically have a density of approximately 85% after sintering. Some of the methods include hot forging, double pressing, double sintering, hot isostatic pressing (xe2x80x9cHIPingxe2x80x9d) and pressure assisted sintering (xe2x80x9cPASingxe2x80x9d). While higher densities (typically, 88% to 92%) are achievable by these methods, it is often at the cost of dimensional precision. And, there is the additional cost of those secondary processes. The blanks need further machining in order to make them into blanks ready for their inside diameter (xe2x80x9cI.D.xe2x80x9d) profiles. Typically, the outside diameters (xe2x80x9cO.D.xe2x80x9d) need to be brought within specifications (the ends need to be squared off and the outside surface ground down) and then the pilot hole running down the center of the blank needs to be made to a specific diameter and concentric to the O.D. The result is referred to as a xe2x80x9csemi-finishedxe2x80x9d blank, which is ready to be made into a finished die.
Making the finished die involves cutting the I.D. profile into the blank. This is done by various means such as drilling, reaming, grinding, EDMing, etc. Tungsten carbide is very hard, so it is difficult (time-consuming and/or costly) to cut in the I.D. profile. The difficulty increases with the complexity of the I.D. profile, the tolerances that must be met and the hardness of the tungsten-carbide blank. Frequently, blanks with lower hardness and/or density are selected in order to overcome or reduce these difficulties.
The present invention provides improved tungsten-carbide dies, with improved physical properties, improved chemical properties and enhanced performance, and an improved method of manufacturing those dies. This invention relates to both the blanks and the finished dies as well as other fastener industry tools.
The present invention produces improved tungsten-carbide blanks and finished dies using MIM. MIM is an established manufacturing process. Heretofore, fine powdered metals (typically spherically-shaped) are mixed with various binders to form a feedstock. This feedstock is then heated and molded under pressure in an injection molding machine to produce a xe2x80x9cgreenxe2x80x9d part or preform. After molding, the binders are removed from the green part in a process called xe2x80x9cdebinding,xe2x80x9d producing a xe2x80x9cbrownxe2x80x9d part or preform. The debound part is then sintered, which fuses the powdered metal particles into a densified matrix. While there is porosity in an MIM part, substantially higher densities are achievable by MIM than by PM. However, we have found that significantly improved results are obtained by using polygonal-shaped powder instead of spherical, oblong, or shard-shaped particles, as defined in Powder Metallurgy Science by Randall M. German, 1994, Chapter 2 and pages 29 and 30, which are herein incorporated by reference.
The green part shrinks substantially during debinding and sintering (typically between 11% and 30%, depending upon the formula of the feedstock and the debinding and sintering parameters). The shrinkage amount, however, is predictable in all dimensions and, once the optimum feedstock formula and parameters are determined, the process is highly consistent and repeatable. The amount of shrinkage that occurs (which is expressed as a percentage equal to one minus the ratio of the size of the finished part to the size of the green part) is referred to as the xe2x80x9cshrink factorxe2x80x9d and the amount by which the green part must be xe2x80x9cover-sizedxe2x80x9d in order to produce a sintered part of specified dimensions (which is expressed as a percentage that is approximately equal to the ratio of the size of the finished part to the size of the green part) is referred to as the xe2x80x9cform factor.xe2x80x9d
Once an appropriate tungsten-carbide feedstock is developed, and its shrink factors and form factors are determined, a mold is fabricated. The mold will produce a blank or finished die with a specified O.D. and length. A pin or pins is then fabricated to be suspended in the mold cavity, which will form the pilot hole (for a blank) or the I.D. profile (for a finished die). Both the mold cavity and the pin(s) are over-sized to take into account the shrinkage that will occur during debinding and sintering. The feedstock is then molded around the pin(s). When the pin or pins are removed, the pilot hole or I.D. profile has been formed in the green part, and when that green part has been debound and sintered, the blank or finished die has been produced with near net shape.
Producing tungsten-carbide dies by this method offers many advantages. Eliminating most if not all of the secondary operations to produce the blanks and the finished dies saves time and expense. In addition, the dies themselves have improved characteristics. The metal powders used to make tungsten-carbide MIM feedstocks are in the present invention polygonal powders. This produces substantially higher densities in the metal (in excess of 99%, compared to 85% by PM) without the need for secondary processes. The polygonal powders also produce an improved microstructure of the metal, with more uniform bonding. This results in increased transverse rupture strength, which is a widely-accepted method used to determine load-bearing properties. The polygonal powders also make it easier to cut in the I.D. profiles into the blanks than the shard-shaped powders used in PM. This allows the use of harder grades of tungsten-carbide to make the same die. All of these improvements result in enhanced performance and/or utility of the die. One additional benefit of these dies is that, when the die wears so that it is no longer within required tolerances, it can easily be reamed to a larger I.D. and re-used.